This is a prospective 1.5-year longitudinal study of the course of illness in 100 subjects with chronic fatigue syndrome (CFS) or chronic fatigue (CF). CFS is characterized by fatigue, disability, and multiple physical and psychological symptoms. It is generally associated with a prolonged course. Subjects with CF have chronic fatigue with insufficient additional symptoms to fulfill CFS criteria. The goals of the study are to improve understanding of which factors are associated with recovery versus worsening of CFS/CF. The first specific goal of this study is to examine psychosocial factors hypothesized to be related to prognosis, i.e., a history of threatening life events (including a history of physical/sexual victimization), psychiatric illness and subclinical psychiatric symptoms, dysfunctional personality characteristics, coping style, social support and reactions of significant others in the patient's environment to the CFS/CF. The second goal of this Project is to examine the interaction effects of psychosocial variables and immune function in predicting the course of illness in CFS/CF.